Calixarenes and calixarene-based sensors

ABSTRACT

A calixarene dimer of the general formula (I-G), comprising a first calixarene moiety I and a second calixarene moiety G, wherein: L is [—CH 2 —] or [—O—CH 2 —O—] and is the same or different between each aryl group; R 5  is H, NO 2 , halogen, or C 1 -C 10  aliphatic hydrocarbyl group, C 6 -C 20  aryl group, C 6 -C 20  hydrocarbylaryl group, any of which is optionally substituted by one or more halo or oxo groups or interrupted by one or more oxo or amide groups, and R 5  is the same or different on each aryl group; R 1  comprises a carboxy group which is or is not protonated or protected; two groups out of R 2 , R 3  and R 4  are H; the one group out of R 2 , R 3  and R 4  not being H comprises at least one atom of one or more of O and S, the said at least one atom being capable of causing the calixarene to be adsorbed onto the surface of the substrate; and the one group out of R 2 , R 3  and R 4  not being H being conjugated to the second calixarene moiety G. The calixarene dimers may be incorporated into sensors. Methods of making the calixarene dimers are disclosed.

[0001] This invention relates to calixarene dimers and their use in thefield of sensors, in particular sensors suitable for use inelectrochemical analysis.

[0002] Sensors have been produced for the detection and measurement ofmany metal species in solution. However, there are currently nosatisfactory sensors for the detection and measurement of uranium andother heavy metals in solution. Senkyr et al. (Analytical Chem., Vol.51, No. 7, p.786, 1979) utilised several acyclic ligands in polymericmembrane electrodes to produce a uranium sensor. Johnson et al (Analyst,Vol. 114, p.1025, 1989) used similar ligands and several cyclic ones inthe development of other sensors based on polymeric membrane ionselective electrodes. While some of these sensors were found to beselective as against other ionic species, it was found that the sensorswere not highly sensitive. Sensors capable of detecting the presence ofGroup I metals utilising calixarenes dispersed within polymericelectrodes have been reported in EP049063 1. Furthermore, U.S. Pat. No.5,705,620 discloses sensors capable of detecting calcium ions, thesensors comprising calixarene moieties immobilised in a polymericmembrane. However, none of the calixarene-based sensors were shown asbeing capable of detecting uranium or other heavy metals. WO97/17322discloses that calix[4]arenes may be linked together to form a dimerspecies, but the properties of the dimers are not disclosed or discussedin any detail. The present invention provides novel calixarenes andsensors using calixarenes, particularly for the detection of uranium andother heavy metals in solution, those sensors giving good sensitivity.

[0003] In accordance with the present invention, a calixarene dimer ofthe general formula I-G comprising a first calixarene moiety of generalformula I and a second calixarene moiety of formula G,

[0004] wherein:

[0005] L is [—CH₂—] or [—O—CH₂—O—] and is the same or different betweeneach awl group;

[0006] R⁵ is H, NO₂, halogen, or C₁-C₁₀ aliphatic hydrocarbyl group,C₆-C₂₀ aryl group, C₆-C₂₀ hydrocarbylaryl group, any of which isoptionally substituted by one or more halo or oxo groups or interruptedby one or more oxo or amide groups, and R⁵ is the same or different oneach aryl group;

[0007] R¹ comprises a carboxy group which is or is not protonated orprotected;

[0008] two groups out of R², R³ and R⁴ are H;

[0009] the one group out of R², R³ and R⁴ not being H comprises at leastone atom of one or both of O and S, the said at least one atom beingcapable of causing the calixarene to be adsorbed onto the surface of thesubstrate; and

[0010] the one group out of R², R³ and R⁴ not being H is conjugated tothe second calixarene moiety, G.

[0011] This provides an ionophore capable of chelating heavy metal ionssuch as uranium and cadmium. Furthermore, the molecule can be readilyadsorbed onto the surface of a substrate, thus allowing sensors to bemade.

[0012] The one group of R², R³ and R⁴ not being H preferably comprisesany one of amide and thioamide. These groups facilitate the simplemanufacture of ionophores. It is further preferred that R² and R⁴ are Hand R³ comprises any one of amide and thioamide.

[0013] It is preferred that the first calixarene moiety is of a formula(II) (not shown), wherein the one group of R², R³ and R⁴ not being Hconforms to the general formula (A):

(A)[—X—Y—S—]

[0014] wherein X is any one of

[0015] R and Y being the same or different and being C₁-C₁₀ aliphatichydrocarbyl group, C₆-C₁₀ aryl group or C₆-C₂₀ hydrocarbylaryl group,any of which may be optionally substituted by one or more halo or oxogroups or interrupted by one or more oxo or amide groups;

[0016] R′ is H, C₁-C₁₀ aliphatic hydrocarbyl group, C₆-C₁₀ aryl group orC₆-C₂₀ hydrocarbylaryl group, any of which may be optionally substitutedby one or more halo or oxo groups or interrupted by one or more oxo oramide groups;

[0017] wherein S is conjugated to the second calixarene moiety G.

[0018] For the avoidance of confusion, it is hereby defined that S is asulphur moiety.

[0019] This provides convenient methods of attaching sulphur-bearingmoieties to the ionophore.

[0020] Alternatively, the first calixarene moiety is of formula (III)(not shown), wherein the one group of R², R³ and R⁴ not being H conformsto the general formula (E):

(E) [—X—Y—S—]

[0021] wherein X is any one of

[0022] R is (C.R²⁰.R²¹)_(m), wherein m is 0, 1, 2 or 3 and R²⁰ and R²¹are H, halogen or C₁-C₁₀ aliphatic hydrocarbyl group and is the same ordifferent on each carbon.

[0023] Y is C₁-C₁₀ aliphatic hydrocarbyl group, C₆-C₁₀ aryl group orC₆-C₂₀ hydrocarbylaryl group, any of which may be optionally substitutedby one or more halo or oxo groups or interrupted by one or more oxo oramide groups;

[0024] R′ is H, C₁-C₁₀ aliphatic hydrocarbyl group, C₆-C₁₀ aryl group orC₆-C₂₀ hydrocarbylaryl group, any of which may be optionally substitutedby one or more halo or oxo groups or interrupted by one or more oxo oramide groups;

[0025] wherein S is conjugated to the second calixarene moiety G.

[0026] For the avoidance of confusion, it is hereby defined that S is asulphur moiety.

[0027] It is further preferred that the second calixarene G is also ofthe general formula (II) or (III).

[0028] The S group of the first calixarene may be conjugated to the Sgroup of the second calixarene, optionally through a spacer group, theoptional spacer group being C₁-C₆ aliphatic hydrocarbyl group, C₆-C₁₀aryl group, C₆-C₁₆ hydrocarbylaryl group any of which may be optionallysubstituted by one of more halo or oxo groups or interrupted by one ormore oxo or amide groups.

[0029] The dimers produced in this manner produce very good sensors.

[0030] It is preferred that X is (CH₂)CONH and Y is an aliphatichydrocarbyl group. This choice of X gives strong chelation between theionophore and the metal ion. Y is most preferably a methyl or an ethylgroup. It is believed that a short Y (and/or short X) group improves theperformance of the resulting sensor. It is preferred that the S group ofthe first calixarene is conjugated directly to the S group of the secondcalixarene, thus forming a disulphide bridging group. This gives goodadhesion to a gold substrate.

[0031] In a preferred embodiment, the calixarene is of formula (V)

[0032] wherein

[0033] L is [—CH₂—] or [—O—CH₂—O—] and is the same or different betweeneach aryl group;

[0034] R⁵ is H, NO₂, halogen, or C₁-C₁₀ aliphatic hydrocarbyl group,C₆-C₂₀ aryl group, C₆-C₂₀ hydrocarbylaryl group, any of which isoptionally substituted by one or more halo or oxo groups or interruptedby one or more oxo or amide groups, and R⁵ is the same or different oneach aryl group;

[0035] R¹ is the same or different on each calixarene moiety comprises acarboxy group which is or is not protonated or protected;

[0036] two groups out of R², R³ and R⁴ on each calixarene moiety are H;

[0037] the one group out of R², R³ and R⁴ not being H on each calixarenemoiety is the respective one of R³¹ and R³³;

[0038] R³¹ and R³³ are the same or different and are C₁-C₁₀ aliphatichydrocarbyl group, C₆-C₁₀ aryl group or C₆-C₂₀ hydrocarbylaryl group,any of which may be optionally substituted by one or more halo or oxogroups or interrupted by one or more oxo or amide groups; or(C.R²⁰.R²¹)_(m),

[0039] wherein m is 0, 1, 2 or 3 and R²⁰ and R²¹ are H, halogen orC₁-C₁₀ aliphatic hydrocarbyl group and is the same or different on eachcarbon;

[0040] R³⁵ and R³⁶ are the same or different and are C₁-C₁₀ aliphatichydrocarbyl group, C₆-C₁₀ aryl group or C₆-C₂₀ hydrocarbylaryl group,any of which may be optionally substituted by one or more halo or oxogroups or interrupted by one or more oxo or amide groups;

[0041] R³² and R³⁴ are the same or different and are C₁-C₁₀ aliphatichydrocarbyl group, C₆-C₁₀ aryl group or C₆-C₂₀ hydrocarbylaryl group,any of which may be optionally substituted by one or more halo or oxogroups or interrupted by one or more oxo or amide groups;

[0042] X′ on each calixarene moiety are the same or different, and are Oor S moieties; and

[0043] R³⁷ is an optional spacer group, which when present is C₁-C₆aliphatic hydrocarbyl group, C₆-C₁₀ aryl group or C₆-C₁₆ hydrocarbylarylgroup any of which maybe optionally substituted by one of more halo oroxo groups or interrupted by one or more oxo or amide groups.

[0044] It is preferred that R³¹ and R³³ are mutually the same. It isalso preferred that R³² and R³⁴ are mutually the same. It is furtherpreferred that R³⁵ and R³⁶ are mutually the same. It is preferred thatone or both of R³¹ and R³³ are conjugated to R³ of their respectivecalixarene moieties. It is preferred that R³¹, R³³, R³² and Re arerelatively short. If any of R³¹, R³², R³³ and R³⁴ are C₁-C₁₀ aliphatichydrocarbyl groups, then it is preferred that these groups are C₁-C₅aliphatic hydrocarbyl groups, most preferred that these groups are C₁-C₃aliphatic hydrocarbyl groups.

[0045] It is preferred that L is [—CH₂—] between each of the aryl groupsand that R⁵ is a tertiary butyl group. It is most preferred that R⁵ isan electron-withdrawing group, such as NO₂.

[0046] The carboxy group R¹ may conform to the general formula (B):

(B) [—Z—COOR¹⁰]

[0047] wherein Z is a C₁, a C₂ or a C₃ carbon chain which is a part ofan aliphatic hydrocarbyl group, aryl group or hydrocarbylaryl group, anyof which is optionally substituted by one or more halo, oxo or nitrogroups; and R¹⁰ is H or a protecting group being a salt or an estergroup. It is preferred that R¹⁰ is H and the aliphatic hydrocarbylgroup, aryl group or hydrocarbylaryl group of formula (B) aresubstituted by one or more groups which cause a reduction in the pKa ofthe carboxylic acid group with respect to an unsubstituted molecule,This increases the ability of the ionophore to complex with heavymetals.

[0048] Alternatively, R¹ is of the general formula (C):

(C) [—(C.R⁶.R⁷)_(n)—COOR¹⁰]

[0049] wherein n is 1, 2 or 3 and R⁶ and R⁷ are H or halogen and is thesame or different on each carbon; and R¹⁰ is H or a protecting groupbeing a salt or an ester group.

[0050] In a further alternative embodiment, R¹ is of the general formula(D):

[0051] wherein n is 0 or 1 and R⁶ and R⁷ are H or halogen and is thesame or different on each carbon and wherein the phenyl ring of thebernzoic acid group is optionally substituted by one or more halo, oxoor nitro groups; and R¹⁰ is H or a protecting group being a salt or anester group. It is preferred that R¹⁰ is H and the phenyl ring of thebenzoic acid of formula (D) is substituted by one or more groups whichcause a reduction in the pKa of the carboxy group with respect to anunsubstituted molecule.

[0052] When R¹ is of the formula (C) or (D), then it is preferred that nis 1 and R⁶ and R⁷ are both H. This gives an acid group with a strongaffinity for heavy metal ions, the chelating oxygen atom being in a goodposition for chelation due to n being 1.

[0053] As a further alternative, some or all of phenyl groups of thecalixarene ring are further peripherally substituted.

[0054] In accordance with a second aspect of the present invention asensor comprises a calixarene dimer in accordance with the presentinvention. It is preferred that the sensor further comprises a substratewherein the calixarene is adsorbed onto the surface of the substrate.This provides a sensor sensitive to low levels of heavy metals, whereinthere is direct molecular contact between the electrode and the activeionophore.

[0055] The substrate may comprise one or more metals, preferably gold.This provides an inexpensive and effective sensor. Gold allows goodadsorption of ionophores onto the surface of the electrode.

[0056] In accordance with a third aspect of this invention, a method forsequestering metals comprises contacting the metals with a calixarenedimer in accordance with the present invention. This provides a good wayof removing even low levels of metals from solution. The method ispreferably carried out at a pH of between about 2 and about 11. The pHis preferably buffered.

[0057] In a most preferred embodiment of this aspect of the invention,the method comprises:

[0058] (i) dissolving the calixarene in an hydrophobic organic solvent;

[0059] (ii) mixing the organic solvent with an aqueous phase containingmetal ions;

[0060] (iii) agitating the organic solvent and aqueous phase together;and

[0061] (iv) recovering the metal from the organic phase.

[0062] This allows the extraction of low levels of heavy metal fromaqueous solution. The metal is preferably selected from any one of U,Cd, Sr, Ca, a Lanthanide and Lu.

[0063] In accordance with a fourth aspect of the present invention, aprocess for preparing a calixarene dimer comprising the use ofcystariline dihydrochloride to conjugate two calixarene moieties.

[0064] The invention will now be described by way of example only withreference to the following figures, of which:

[0065]FIG. 1 is a reaction scheme for the production of calixarenedimers in accordance with the present invention;

[0066]FIG. 2 is a reaction scheme for the production of more calixarenedimers in accordance with the present invention;

[0067]FIG. 3 is a schematic representation of the mechanism by which itis expected that calixarenes deployed in sensors according to thepresent invention undergo reduction and oxidation;

[0068]FIG. 4 is a voltamogram obtained from a sensor in accordance withthe present invention, the sensor not being in contact with a testsolution;

[0069]FIG. 5 is a graph showing the current peak height in a voltamogramas a function of the concentration of uranium ions as measured by asensor in accordance with the present invention;

[0070]FIG. 6 is a graph showing the current peak height in a voltamogramas a function of the scan rate as measured by a sensor in accordancewith the present invention;

[0071]FIG. 7 is a graph showing the current peak height in a voltamogramas measured by a sensor in accordance with the present invention as afunction of extraction time for a variety of metal ions;

[0072]FIG. 8 is a cyclic voltamogram generated using a sensor inaccordance with the present invention;

[0073]FIG. 9 is a graphical representation of the current peak heightfrom a sensor in accordance with the present invention as a function oftime and uranium ion concentration; and

[0074]FIG. 10 is a graphical representation of the current peak heightfrom a sensor in accordance with the present invention as a function ofthe square root of the scan rate.

[0075] Reaction Scheme 1

[0076] Dimers 6 and 7 in accordance with the present invention aresynthesised in accordance with the reaction scheme shown in FIG. 1.

[0077] Calix[4]arene 8 was prepared by standard procedures via thedebutylation of tertiary butyl calix[4]arene with aluminium chloride(Arduini and Casnati, Macrocyle Synthesis, Ed. David Parker, 1995,Oxford).

[0078] Synthesis of 2

[0079] A mixture of calix[4]arene 8 (1.0 g, 2.35 mmol), potassiumcarbonate (0.71 g, 5.17 mmol), acetone (50 cm³) and bromoethylacetate(0.57 cm³, 5.17 mmol) was stirred at room temperature, under nitrogen,for 6 days. The mixture was then evaporated to dryness, then slurriedwith ethyl acetate and hexane, filtered and the filtrate evaporated todryness. This filtrate was then slurried with DCM and filtered and thefiltrate evaporated to dryness, then recrystailised from ethyl acetateand chloroform giving 2 (0.33 g), m.p., 183° C. The structure of 2 wasverified by NMR and mass spectroscopy.

[0080] Synthesis of 3

[0081] A mixture of 2 (3.93 g, 6.6 mmol), ethanol (240 cm³) andpotassium hydroxide (28.6 cm³ of a 0.46 M solution in ethanol, 13.2mmol) was stirred at reflux for 2.5 h. The mixture was then evaporatedto dryness and dried in a vacuum oven at 100° C. for 2 h, giving asolid.

[0082] Preparation of 4

[0083] DCM (100 cm³) and dilute hydrochloric acid (50 cm³) were added to3 obtained above, and the mixture shaken. The mixture was allowed tosettle and the DCM layer separated and evaporated to dryness. Next,acetic acid and ethanol were added to the aqueous layer and mixturefiltered and the solids combined with the DCM soluble solids (3.66 g).The solid was then purified on silica, eluting with DCM, hexane andacetic acid (2:2:1), giving 4 (0.80 g), m.p., 244° C. The structure of 4was verified by NMR and mass spectroscopy.

[0084] Synthesis of 5

[0085] A mixture of 4 (0.50 g, 0.88 mmol), DCM (10 cm³) and thionylchloride (1.0 cm³) was stirred under nitrogen, under reflux for 3 h. AnIR spectrum of the mixture confirmed that 4 had been converted to theacid chloride ester, 5, υ_(max) 1809 (COCl), 1752 (CO₂ Et). Next, DCMand thionyl chloride were removed by distillation under reducedpressure, giving crude 5, which was used without further purification.

[0086] Synthesis of 6

[0087] The acid chloride 5 (0.50 g, 0.88 mmol) was dissolved in DCM (10cm³) and added to cystamine hydrochloride (0.10 g, 0.44 mmol) andtriethylamine (0.5 cm³), under nitrogen, with siring. After a fewminutes, the mixture rapidly darkened. The reaction was monitored by thedisappearance of the acid chloride peak and after 3 days, the mixturewas evaporated to dryness. This solid was then slurried with hotethylacetate and filtered, giving a solid, assumed to be unreactedcystamine hydrochloride and triethylamine hydrochloride (0.34 g). Thefiltrate was evaporated to dryness (0.43 g), then column chromatographyon silica, eluting DCM and hexane gave unreacted 4 (0.30 g) and 6 (0.02g, 2%). The structure of 6 was verified by NMR, IR and massspectroscopy.

[0088] Synthesis of 7

[0089] Reaction of the acid chloride 6 with ethanolic potassiumhydroxide did not yield the expected acid product 7. However, thoseskilled in the art will recognise that other agents, such as Ba(OH)₂ areavailable to perform this reaction.

[0090] Reaction Scheme 2

[0091] Dimers 1A and 1B in accordance with the present invention aresynthesised in accordance with the reaction scheme shown in FIG. 2.

[0092] Synthesis of 1A

[0093] The acid chloride 1C (analogous to 5 above) maybe produced usingthe general methodology described above for the manufacture of 5, butusing tertiary butyl calix[4]arene as a starting material, instead ofcalix[4]arene. The acid chloride 1C (0.5 g, 0.63 mmol) was added to amixture of triethylamine (0.3 cm³), cystamine dihydrochloride (0.071 g,0.3 1 mmol) and dichloromethane (20 cm³) with stirring, under nitrogenat room temperature. The mixture was then stirred at room temperaturefor 18 hours. Water (20 cm³) and dilute hydrochloric acid (20 cm³) wereadded and the mixture extracted into dichloromethane. Thedichloromethane was then evaporated to give a solid (0.75 g). Columnchromatography on silica, eluting cyclohexane:ethylacetate (3:2) gaveionophore 1A (0.1 g). The structure was confirmed by infra-red (IR) andnuclear magnetic resonance (NMR) spectroscopy. Note that the synthesisof the acid chloride IC is disclosed in WO97/17322.

[0094] Synthesis of 1B

[0095] A mixture of 1A (0.14 g, 0.08 mmol), ethanol (20 cm³) andethanolic potassium hydroxide solution (0.50 cm³ of 0.43M solution) wasrefluxed for 2 days. The mixture was cooled and acidified with dilutehydrochloric acid. The resulting solid was filtered from the suspensionand dried (0.025 g). The solid was recrystallised from a mixture ofdichloromethane and methanol, giving partially purified 1B. Thestructure of 1B was confirmed by IR and NMR spectroscopy. Small amountsof impurities were found in the partially purified 1B.

[0096] Despite this patent application disclosing the synthesis of onlya limited number of calixarenes in accordance with the presentinvention, those skilled in the art will realise that the claimedmolecules can be readily synthesised using the teaching of this documentin combination with that of WO97/17322. WO97/17322, inter alia, teacheshow to modify the periphery of the calixarene moiety.

[0097] It is anticipated that the thioamide analogues of 1A and 6 can besynthesised by refluxing 1A and 6 respectively with Lawesson's reagent.Furthermore, it is anticipated that one can readily synthesise dimerswhere calixarene moieties I and G are not the same. The moieties I and Gwould each contain reactive acid chloride groups. I and G would then bereacted with a trialkylamine and cystamine dihydrochloride to formdimers, as outlined in the synthesis of 1A and 6 above. Three dimerstructures would be formed, and these could be separated if desired.

[0098] These ionophores can be readily used in sensors as is nowdiscussed.

[0099] A sensor in accordance with the present invention comprises acalixarene in accordance with the present invention. It is preferredthat the calixarene is adsorbed onto the surface of an electrode. Theterm ‘adsorbed’ is intended to include chemically adsorbed andphysically adsorbed. There are several ways of depositing the calixareneonto the surface of the electrode, such as spin coating, LangmuirBlodgett deposition and plasma coating. However, the easiest manner ofmaking such a sensor is to immerse the electrode into a solution of thecalixarene. This causes a nominal monolayer of the calixarene to form onthe surface of the electrode. The resulting electrode is often referredto as a chemically modified electrode. The calixarene is adsorbed ontothe surface of the electrode but it is unclear as to whether the layerformed is a true monolayer.

[0100] The sensor can then be characterised by cyclic voltammetry (CV),a technique well-known to those skilled in the art. A description ofthis technique can be found in “Instrumental methods inelectrochemistry” by Greef, Peat, Peter, Pletcher and Robinson,published by Ellis Horwood.

[0101] The sensor is immersed in a suitable ionic solution and CV usedto probe the interaction between the sensor and ions. Two peaks willtypically be observed in the CV cycle. One of these represents oxidationof a component of the sensor and the other represents the correspondingreduction reaction. The nature, and quantity, of the ion adsorbed by theionophore may effect the redox characteristic of the said component. Forexample, the chelation of different ions has been found to alter thespacing between the two peaks, and increasing concentrations of ionshave been found to increase the height of the peaks. Hence, such sensorsin accordance with the present invention have been found to provideexcellent performance.

[0102] Examples of the manufacture and characterisation of sensors inaccordance with the present invention are now described.

EXAMPLE 1

[0103] A gold electrode with an exposed surface area of 1-2 mm2 wasimmersed for 15 minutes in a 0.2 mM solution of the calixarene 1B (FIG.2) in dichloromethane. It is anticipated that the interaction betweenthe sulphur atoms of the thiol group and the gold atoms of the electrodecause the ionophore to be adsorbed onto the electrode surface as isillustrated schematically in FIG. 3. The interaction between the sulphurand gold atoms is strong, yet the exact nature of the interaction isunknown

[0104] This sensor was characterised using CV. An estimate of 30%monolayer coverage was determined by measuring the reduction in area ofthe CV curve for the electrode bearing the monolayer film compared tothe electrode bearing no film (FIG. 4).

EXAMPLE 2

[0105] A sensor as produced using the methodology of example 1 wascharacterised by measuring the CV characteristics of the sensor as afunction of uranium ion concentration at pH=2 and with a constantpotential sweep rate of 100 mV/s. The sensor was immersed in the testsolution for 15 minutes prior to taking a reading. The data indicate alinear increase in the current peak height with increase in ionconcentration over the range 100-300 ppb (FIG. 5). However, it isunclear whether the relationship between peak height and ionconcentration is linear below 100 ppb. These data illustrate that asensitive sensor can be easily achieved using the present invention.

EXAMPLE 3

[0106] Another sensor produced using the methodology of example 1 wascharacterised using cyclic voltammetry. The height of the oxidation andreduction peaks are also dependent on the potential sweep rate, with anincrease in sweep rate giving a sub-linear increase in peak height (FIG.6). It is believed that an increased scan rate gives a taller peak dueto an experimental artefact of there being fewer data points accumulatedas the scan rate increases.

EXAMPLE 4

[0107] The responses (current peak height) of sensors produced inaccordance with the methodology of example 1 were monitored as afunction of the time and as a function of cation Each sensor wasimmersed in a solution of one of various cations (U, Pb, Cd, Sr, Ca, Lu,pH=2, 500 ppb solutions) and the CV response was measured, giving theresults shown in FIG. 7. This indicates that in each case there is aperiod for which no response is measured by the sensor i.e. no oxidationor reduction peaks were observed in the CV curves. This period wasshortest for U but was still reasonably long at approximately 9 minutes.U, Sr, Cd, Lu and Ca generated a response in the sensor, but Pb did not.This illustrates that the sensor is somewhat selective, with a greateraffinity for U, Cd and Sr. Furthermore, for each cation there is alinear response in current peak height with increasing extraction time.It was also noted that the difference in voltage between the oxidationand reduction peaks was a function of the ion chelated to the calixarenemolecule as shown below in Table 1. TABLE 1 Positions of the oxidationand reduction peaks for several types of metal ion Position of oxidationPosition of reduction Difference between Analyte peak (mV) peak (mV)peak positions (mV) U 304 86 218 Cd 289 98 191 Ce 287 80 207 Lu 312 91221 Sr 323 24 299 Ca 295 112 183

[0108] Hence, the difference in voltage between the two peaks may beused to identify the ion detected by the sensor. However, the resultsfrom an analysis of a mixture of U, Cd and Ce ions gave results thatwere inconsistent with those shown in Table 1.

EXAMPLE 5

[0109] A layer of compound 1B, a calixarene dimer bridged with adisulphide group, was deposited onto a gold substrate using themethodology of example 1. The sensor was tested in accordance with thegeneral methodology of examples 1 and 2. FIG. 8 shows a cyclicvoltamogram generated using this sensor immersed in an aqueous uraniumion solution. The two peaks, 20, 21 indicate reduction and oxidationprocesses respectively. FIG. 9 shows the current peak height from thesensor as a function of time and the concentration of uranium. Thisdemonstrates that the sensor can detect ultra-low levels of uranium,down to 1 ppb, given sufficient time for the electrode to react Thisalso confirms that the current peak height varies linearly with the timefor which the sensor is exposed to the solution, there being an initialtime period in which the sensor does not respond. The speed of theresponse of the detector (i.e. the rate of change in the current peakheight for a given change in time) increases as the concentration of thesolution increases, although it appears that the relationship betweenspeed of response and concentration is sub-linear.

[0110] The current peak height was measured as a function of thepotential scan rate as shown in FIG. 10. Note that the current peakheight increases as the scan rate increases in a sub-linear manner. Thisis thought to be an artefact of the experimental set-up and isconsistent with the results of example 3.

[0111] It is not known whether the calixarene dimer 1B remains intactwhen deposited onto the surface of the electrode. It is quite possiblethat the disulphide bridge breaks, giving 2 monosulphide or thiolmonomers.

[0112] The excellent performance of the calixarene-based sensors issurprising: it has been shown by Beer (Inorganic Chemistry, 1997, 36,5880) that chemical oxidation of p-tert-butylcalix[4]arene bis estersand amides gives p-tert-butylcalix[4]diquinones, and thesecalix[4]diquinones have been shown to display anodic shifts in theirreduction potentials in the presence of group 1 or 2 metal, ammonium andalkylammonium complexes. However, electrochemical oxidationofp-tert-butylcalix[4]arenes in the presence or absence of metal cationshas not been observed or considered in the literature (for example, seechapter 7 in “Calixarenes Revisited” by C. David Gutsche or “Calixarenesin Action” by Luigi Mandolini and Rocco Ungaro and references therein).

[0113] It was thought that one had to incorporate additional moietiesinto the system, those moieties readily undergoing redox reactions andbeing placed in electrostatic proximity to the ionophore (Paul D. Beer,J. Chem Soc., Dalton Trans., 1999, 1897). In such a system, thecomplexation of a metal ion (uranium, for instance) into the calixarenemolecule would alter the redox characteristics of the nearby addedmoiety and hence the system would act as a sensor.

[0114] The reasons for the unexpected excellent performance of thesensors in accordance with the present invention are unclear but may berelated to the relatively high density of calixarene ions on the surfaceof the detector and the relatively short distance between the calixarenegroup and the substrate. As mentioned above, it was previously thoughtthat the calixarene moiety was inert to redox reactions, even when itincorporated metal ions. However, it has been demonstrated that acomponent of the sensors in accordance with the present invention isinvolved in a redox reaction. It is herein tentatively proposed that theredox activity of the sensors can be explained by the schematicrepresentation of FIG. 3. The proximity of both the uranyl ion and thegroup undergoing redox reactions to the electrode surface probablycontribute to the superior performance of the sensors in accordance withthe present invention.

[0115] Notwithstanding the correctness of the above interpretation ofthe mechanism of the redox reaction, it has been shown that sensorscomprising calixarenes adsorbed onto the surface of an electrode arereadily achievable.

[0116] It is clear from the present patent application that thecalixarene dimers in accordance with the present invention are capableof extracting certain metal ions from solution. It is therefore to beexpected that these dimers can be readily used in the extraction methodsfound in WO97/17322.

1. A calixarene dimer of the general formula I-G comprising a firstcalixarene moiety of general formula I and a second calixarene moiety G,

wherein: L is [—CH₂—] or [—O—CH₂—O—] and is the same or differentbetween each aryl group; R⁵ is H, NO₂, halogen, or C₁-C₁₀ aliphatichydrocarbyl group, C₆-C₂₀ aryl group, C₆-C₂₀ hydrocarbylaryl group, anyof which is optionally substituted by one or more halo or oxo groups orinterrupted by one or more oxo or amide groups, and R⁵ is the same ordifferent on each aryl group; R¹ comprises a carboxy group which is oris not protonated or protected; two groups out of R², R³ and R⁴ are h;the one group out of R², R³ and R⁴ not being h comprises at least oneatom of one or both of O and S, the said at least one atom being capableof causing the calixarene to be adsorbed onto the surface of thesubstrate; and the one group out of R², R³ and R⁴ not being h isconjugated to the second calixarene moiety, G.
 2. A dimer according toclaim 1 wherein the one group of R², R³ and R⁴ not being H comprises anyone of thioamide and amide.
 3. A dimer according to claim 2 wherein R²and R⁴ are H and R³ comprises any one of thioamide and amide.
 4. A dimeraccording to any one of claims 1 to 3, wherein the structure of firstcalixarene moiety is known as formula (II), the one group of R², R³ andR⁴ not being H conforming to the general formula (A): (A) [—X—Y—S]wherein X is any one of

R and Y being the same or different and being C₁-C₁₀ aliphatichydrocarbyl group, C₆-C₁₀ aryl group or C₆-C₂₀ hydrocarbylaryl group,any of which may be optionally substituted by one or more halo or oxogroups or interrupted by one or more oxo or amide groups; R′ is H,C₁-C₁₀ aliphatic hydrocarbyl group, C₆-C₁₀ aryl group or C₆-C₂₀hydrocarbylaryl group, any of which may be optionally substituted by oneor more halo or oxo groups or interrupted by one or more oxo or amidegroups; wherein S is conjugated to the second calixarene moiety G.
 5. Adimer according to any one of claims 1 to 3 wherein the structure of thefirst calixarene moiety is known as formula (E), the one group of R², R³and R⁴ not being H conforming to the general formula (E): (E)[—X—Y—S—]wherein X is any one of

R is (C.R²⁰.R²¹), wherein m is 0, 1, 2 or 3 and R²⁰ and R²¹ are H,halogen or C₁-C₁₀ aliphatic hydrocarbyl group and is the same ordifferent on each carbon. Y is C₁-C₁₀ aliphatic hydrocarbyl group,C₆-C₁₀ aryl group or C₆-C₂₀ hydrocarbylaryl group, any of which may beoptionally substituted by one or more halo or oxo groups or interruptedby one or more oxo or amide groups; R′ is H, C₁-C₁₀ aliphatichydrocarbyl group, C₆-C₁₀ aryl group or C₆-C₂₀ hydrocarbylaryl group,any of which may be optionally substituted by one or more halo or oxogroups or interrupted by one or more oxo or amide groups; wherein S isconjugated to the second calixarene moiety G.
 6. A dimer according toany preceding claim wherein the second calixarene moiety G is of thegeneral formula (II) or (III).
 7. A dimer according to claim 6 whereinthe S group of the first calixarene moiety is conjugated to the S groupof the second calixarene moiety through a spacer group, the spacer groupbeing C₁-C₆ aliphatic hydrocarbyl group, C₆-C₁₀ aryl group, C₆-C₁₆hydrocarbylaryl group any of which may be optionally substituted by oneof more halo or oxo groups or interrupted by one or more oxo or amidegroups.
 8. A dimer according to any one of claims 1 to 6 wherein the Sgroup of the first calixarene is conjugated directly to the S group ofthe second calixarene.
 9. A dimer according to any one of claims 4 to 8wherein X is (CH₂)CONH and Y is an aliphatic hydrocarbyl group.
 10. Adimer according to claim 9 wherein Y is an ethyl group.
 11. A dimeraccording to any one of claims 1 to 10 wherein L is [—CH₂—] between eachof the aryl groups.
 12. A dimer according to any one of claims 1 to 11wherein R⁵ is a tertiary butyl group.
 13. A dimer as claimed in any oneof claims 1 to 12 wherein the carboxy group R¹ conforms to the generalformula (B): (B) [—Z—COOR¹⁰] wherein Z is a C₁, a C₂ or a C₃ carbonchain which is a part of an aliphatic hydrocarbyl group, aryl group orhydrocarbylaryl group, any of which is optionally substituted by one ormore halo, oxo or nitro groups; and R¹⁰ is H or a protecting group beinga salt or an ester group.
 14. A dimer as claimed in claim 13 wherein R¹⁰is H and the aliphatic hydrocarbyl group, aryl group or hydrocarbylarylgroup of formula (B) are substituted by one or more groups which cause areduction in the pKa of the carboxylic acid group with respect to anunsubstituted molecule.
 15. A dimer as claimed in any one of claims 1 to12 wherein R¹ is of the general formula (C): (C)[—(C.R⁶.R⁷)_(n)—COOR¹⁰]wherein n is 1, 2 or 3 and R⁶ and R⁷ are H orhalogen and is the same or different on each carbon and R¹⁰ is H or aprotecting group being a salt or an ester group.
 16. A dimer as claimedin any one of claims 1 to 12 wherein R¹ is of the general formula (D):

wherein n is 0 or 1 and R⁶ and R⁷ are H or halogen and is the same ordifferent on each carbon and wherein the phenyl ring of the benzoic acidgroup is optionally substituted by one or more halo, oxo or nitrogroups; and R¹⁰ is H or a protecting group being a salt or an estergroup.
 17. A dimer as claimed in claim 16 wherein R¹⁰ is H and thephenyl ring of the benzoic acid of formula (D) is substituted by one ormore groups which cause a reduction in the pKa of the carboxy group withrespect to an unsubstituted molecule.
 18. A dimer as claimed in any oneof claims 15 to 17 wherein n is 1 and R⁶ and R⁷ are both H.
 19. Acalixarene as claimed in claim 1 of formula (V)

wherein L is [—CH₂—] or [—O—CH2—O—] and is the same or different betweeneach aryl group; R⁵ is H, NO₂, halogen, or C₁-C₁₀ aliphatic hydrocarbylgroup, C₆-C₂₀ ar group, C₆-C₂₀ hydrocarbylaryl group, any of which isoptionally substituted by one or more halo or oxo groups or interruptedby one or more oxo or amide groups, and R⁵ is the same or different oneach aryl group; R¹ is the same or different on each calixarene moietycomprises a carboxy group which is or is not protonated or protected;two groups out of R², R³ and R⁴ on each calixarene moiety are H; the onegroup out of R², R³ and R⁴ not being H on each calixarene moiety is therespective one of R³¹ and R³³; R³¹ and R³³ are the same or different andare C₁-C₁₀ aliphatic hydrocarbyl group, C₆-C₁₀ aryl group or C₆-C₂₀hydrocarbylaryl group, any of which may be optionally substituted by oneor more halo or oxo groups or interrupted by one or more oxo or amidegroups; or (C.R²⁰.R²¹)_(m), wherein m is 0, 1, 2 or 3 and R²⁰ and R²¹are H, halogen or C₁-C₁₀ aliphatic hydrocarbyl group and is the same ordifferent on each carbon; R³⁵ and R³⁶ are the same or different and areC₁-C₁₀ aliphatic hydrocarbyl group, C₆-C₁₀ aryl group or C₆-C₂₀hydrocarbylaryl group, any of which may be optionally substituted by oneor more halo or oxo groups or interrupted by one or more oxo or amidegroups; R³² and R³⁴ are the same or different and are C₁-C₁₀ aliphatichydrocarbyl group, C₆-C₁₀ aryl group or C₆-C₂₀ hydrocarbylaryl group,any of which may be optionally substituted by one or more halo or oxogroups or interrupted by one or more oxo or amide groups; X′ on eachcalixarene moiety are the same or different, and are O or S moieties;and R³⁷ is an optional spacer group, which when present is C₁-C₆aliphatic hydrocarbyl group, C₆-C₁₀ aryl group or C₆-C₁₆ hydrocarbylarylgroup any of which may be optionally substituted by one of more halo oroxo groups or interrupted by one or more oxo or amide groups.
 20. Acalixarene according to any preceding claim wherein some or all ofphenyl groups of the calixarene ring are further peripherallysubstituted.
 21. A sensor comprising a calixarene dimer as claimed inany one of claims 1 to
 20. 22. A sensor according to claim 21 furthercomprising a substrate, wherein the calixarene is adsorbed onto thesurface of the substrate.
 23. A sensor according to claim 22 wherein thesubstrate comprises one or more metals.
 24. A sensor according to claim23 wherein the substrate comprises gold.
 25. A method of sequesteringmetals comprising contacting the metals with a calixarene dimer asclaimed in any one of claims 1 to
 20. 26. A method according to claim 25wherein the method is carried out at a pH of between about 2 and about11.
 27. A method according to either of claims 25 or 26 wherein the pHat which the method is carried out is buffered.
 28. A method accordingto any one of claims 25 to 27 comprising the following steps: (i)dissolving the calixarene in an hydrophobic organic solvent; (ii) mixingthe organic solvent with an aqueous phase containing metal ions; (iii)agitating the organic solvent and aqueous phase together; and (iv)recovering the metal from the organic phase.
 29. A method according toany one of claims 25 to 28 wherein the metal is selected from any one ofU, Cd, Sr, Ca, a Lanthanide and Lu.
 30. A process for preparing acalixarene dimer as claimed in any one of claims 1 to 20 comprising theuse of cystamine dihydrochloride to conjugate two calixarene molecules.31. A process for preparing a calixarene dimer as claimed in any one ofclaims 1 to 20 substantially as hereinbefore described with reference toFIGS. 1 and 2.